Polyvinyl alcohol (hereinafter, may be abbreviated as PVA) resins are crystalline water-soluble polymer materials and are widely applied to emulsifiers, suspensions, surfactants, fiber treating agents, various binders, paper treating agents, adhesives, films, and the like using their excellent water solubility and film properties (strength, grease resistance, film formability, oxygen gas barrier properties, etc.). Conventionally, PVAs having different degrees of saponification and polymerization are employed depending on the use. Various modified PVAs with special functions by introducing a functional group into PVAs are proposed.
Polyvinyl alcohols are industrially produced by saponification of polyvinyl acetate that is obtained by radically polymerizing vinyl acetate. In radical polymerization reaction of vinyl acetate, various types of side reaction, such as chain transfer reaction and recombination termination reaction, occur during the polymerization, and thus it is generally considered difficult to precisely control the molecular weight distribution, the end structure, and the like of the polyvinyl acetate (and polyvinyl alcohols) thus obtained. For improvement in thermal stability and mechanical properties of polyvinyl alcohols, polymers having a less content of a low molecular weight polymer, that is, polymers having a high molecular weight and a narrow molecular weight distribution are considered preferred.
In recent years, with the advances in the so-called living radical polymerization technique, some methods for controlling radical polymerization reaction of vinyl acetate have been proposed. For example, a method is proposed that comprises radical polymerization reaction of vinyl acetate in the presence of a radical polymerization initiator and a specific control agent to obtain polyvinyl acetate having a narrow molecular weight distribution. In such polymerization reaction, a propagating radical end of the molecular chain of polyvinyl acetate is covalently bonded with the control agent to form dormant species. The polymerization progresses while establishing an equilibrium between the dormant species and radical species generated by dissociation of the dormant species. Such polymerization reaction is referred to as controlled radical polymerization.
However, it used to be difficult to obtain polyvinyl acetate with a high molecular weight by controlled radical polymerization in the past. This is considered to be because a radical is thermally extremely unstable that is generated at an end of a head-to-head bond (a bond having acetyl groups of vinyl acetate adjacent to each other) generated with a certain probability during the polymerization and the equilibrium is thus largely shifted to the dormant species side, resulting in no further progress of the polymerization reaction. In contrast, in a case where the polymerization temperature is raised to promote thermal dissociation of the dormant species, the controllability becomes worse in spite of the progress of the reaction. Accordingly, it has been extremely difficult to obtain polyvinyl acetate with a high molecular weight while maintaining controllability.
Regarding such a problem, Patent Document 1 reports an example in which radical polymerization reaction of vinyl acetate is performed in the presence of a control agent containing a radical polymerization initiator and an iodine compound, thereby synthesizing polyvinyl acetate having a number-average molecular weight (Mn) of 92,000 and a molecular weight distribution (Mw/Mn) of 1.57 and saponifying it to produce a polyvinyl alcohol. However, in the polymerization method using an iodine compound as a control agent, it is known that an aldehyde group is formed at a polymerization end of polyvinyl acetate (e.g., refer to Non-Patent Document 1). In a case of saponifying such polyvinyl acetate having an aldehyde group at an end, it is known that a conjugated polyene structure where plural carbon-carbon double bonds are conjugated is formed to obtain markedly colored polyvinyl alcohols.
Recently, a technique has been proposed to synthesize polyvinyl acetate having a narrow molecular weight distribution and a high molecular weight by controlled radical polymerization using an organic cobalt complex as a control agent. In the polymerization reaction, the propagating radical end of the molecular chain of polyvinyl acetate is covalently bonded with a cobalt atom of an organic cobalt complex to form dormant species. The polymerization progresses while establishing equilibrium between the dormant species and radical species generated by dissociation of the dormant species. For example, Non-Patent Document 2 reports an example in which vinyl acetate is polymerized in the presence of cobalt (II) acetylacetonate, thereby synthesizing polyvinyl acetate having a number-average molecular weight (Mn) of 99,000 and a molecular weight distribution (Mw/Mn) of 1.33.
Non-Patent Document 3 describes that a polyvinyl acetate chain obtained by polymerizing vinyl acetate in the presence of cobalt (II) acetylacetonate is treated with 1-propanethiol. While the polyvinyl acetate chain forms a dormant species bonded with a cobalt (III) complex at an end, an end radical formed by cleavage of the dormant species reacts with 1-propanethiol, thereby allowing separation of the cobalt complex from the polyvinyl acetate chain. While polyvinyl acetate forming the dormant species is green, it is described that the separated cobalt complex is precipitated, followed by filtration through celite for removal to obtain less colored polyvinyl acetate. Instead of 1-propanethiol, TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) as a stable radical compound may be used for bonding of TEMPO with the end radical to trap a radical. It is described that, in this case as well, the cobalt complex is filtered with acidic alumina for removal to obtain colorless polyvinyl acetate.
As just described, according to the method described in Non-Patent Document 3, it is possible to obtain less colored polyvinyl acetate. However, Non-Patent Document 3 does not describe that the polyvinyl acetate thus obtained is subjected to saponification to yield polyvinyl alcohol. As a result of experiments by the present inventors, it was found that the polyvinyl alcohol obtained by saponifying the polyvinyl acetate obtained in accordance with Non-Patent Document 3 was colored. For filtration and sufficient removal of the cobalt complex contained in the polymer solution, the cobalt complex has to be precipitated by appropriately selecting the concentration of the solution and the type of solvent. However, to efficiently precipitate the cobalt complex by filtration, a large amount of solvent is required for dilution and a pressure raise due to the precipitates, flow rate reduction due to the filtration, and the like also occur, and thus productivity turns out to be reduced.